The regulation of blood flow in the various tissues throughout the body is a complicated process involving many factors. Blood pressure is maintained by regulation of cardiac output and peripheral resistance at the arterioles, postcapillary venules, and heart. The kidney contributes to maintenance of blood pressure by regulating blood volume. Central control of blood pressure is integrated and regulated by diffuse neurons within a region of the medulla oblongata loosely called the vasomotor center. Hypoxia or carbon dioxide directly stimulates the vasomotor center. It also receives input from sensors within the walls of the large arteries. Output from the vasomotor center alters heart rate and vascular tone to return blood pressure to acceptable levels.
Tissues can regulate their own blood flow through autoregulation. Local factors such as decreased oxygen, increased carbon dioxide, or increased osmolarity relax arterioles and precapillary sphincters. Lactate and potassium ion concentrations also cause local vasodilation. Injury causes arteries and arterioles to constrict to limit blood loss. Temperature decreases cause vasoconstriction in localized areas as well.
Circulating or local hormonal factors can change the caliber of the arterioles. Histamine, atrial natriuretic peptide, epinephrine, kinins, nitric acid, and adenosine are all vasodilators. Vasoconstrictors include vasopressin, norepinephrine, angiotensin II, and serotonin. The action of any of many of these substances depends on the tissue and receptor compliment of the cells there.
The precise control of blood flow is critical to the proper function of organs and tissue throughout the body and can also play a role in behavior, learning, and memory. Blood flow through the pulmonary circulation is highly regulated. For example, the pulmonary endothelium regulates pulmonary blood flow and maintains a low vascular resistance by releasing vasoactive substances that control vasomotor tone, vascular patency, and normal vessel wall architecture.
Pulmonary hypertension (PH) is a condition of increased pulmonary vascular resistance and pulmonary arterial pressure that interferes with ventilation-perfusion relationships. PH typically is characterized by increased blood pressure (above 30 mm Hg systolic and 12 mm Hg diastolic) within the pulmonary circulation.
There are two subsets of pulmonary hypertension: primary (idiopathic or “unexplained”) and secondary. The secondary form is the more prevalent. The most common causes of secondary pulmonary hypertension are heart disease and lung disease. Regardless of the root cause of the pulmonary hypertension, the vessels of the lungs undergo anatomic changes that contribute to the progression of pulmonary hypertension.
Nitric oxide is one compound that plays an important role in regulating pulmonary blood flow. However, it is a gas with no known storage mechanism, which diffuses freely across membranes and is extremely labile. Nitric oxide has a biological half-life on the order of seconds, and its production is tightly regulated.
Nitric oxide is produced by two classes of nitric oxide synthases (NOS). The constitutively expressed nitric oxide synthases exist as two isoforms: the endothelial nitric oxide synthase (eNOS) and the neuronal nitric oxide synthase, (nNOS). These isoforms are expressed in vascular endothelial cells, platelets, and in neural tissues such as the brain.
In blood vessels eNOS mediates endothelium dependent vasodilation in response to various mediators. Nitric oxide levels increase in response to shear stress, i.e., forces on the blood vessels in the direction of blood flow, and are the mediators of inflammation.
In the nervous system, the neuronal NOS isoform is localized to discrete populations of neurons in the cerebellum, olfactory bulb, hippocampus, corpus striatum, basal forebrain, and brain stem.
The second class of nitric oxide synthase, inducible nitric oxide synthase (iNOS), is expressed in macrophages, hepatocytes, and tumor cells. Steuhr et al., Adv. Enzymol. Relat. Areas Mol. Biol. 65:287-346 (1992); Lowenstein et al., Proc. Natl. Acad. Sci. (USA) 89:6711-6715 (1992). This form of NOS functions as a cytotoxic agent, and NO produced by inducible NOS targets tumor cells and pathogens.
All isoforms of NOS catalyze the conversion of L-arginine to L-citrulline with production of NO. In vascular smooth muscle cells and in platelets, NO activates soluble guanylate cyclase, which increases intracellular guanosine 3′,5′-cyclic monophosphate (cGMP), thereby inducing vasorelaxation and inhibiting platelet aggregation.
Pulmonary arterial hypertension secondary to acquired heart disease begins with a disorder of the left ventricle that leads to pulmonary venous hypertension followed by pulmonary arterial hypertension. The pathobiology of PAH is complex. Vascular endothelial cell dysfunction appears to be a factor in the development of PAH as vascular endothelium-derived mediators including nitric oxide (NO) play a role in the regulation of vascular function through NO/cGMP-mediated vasorelaxation in the pulmonary circulation. Reduced expression of eNOS and/or diminished production of NO in the lungs of patients with pulmonary hypertension have been observed (Giaid A, and D. Saleh (1995) “Reduced expression of endothelial nitric oxide synthase in the lungs of patients with pulmonary hypertension” N. Engl. J. Med. 333:214-221). Vascular endothelial cells generate NO from the metabolism of L-arginine via an oxidative catabolic reaction mediated by eNOS (Michel T. and O. Feron (1997) “Nitric oxide synthases: Which, where, how, and why?” J. Clin. Invest. 100:2146-2152; Garcia-Cardena G, R. Fan, D. F. Stern, J. Liu, and W. C. Sessa (1996) “Endothelial cell nitric oxide synthase is regulated by tyrosinee phosphorylation and interacts with caveolin-1” J. Biol. Chem. 271:27237-27240), which activates soluble guanylate cyclase resulting in increased production of cGMP. Thus, physiologic action of NO is mediated through formation of cGMP or NO/cGMP signaling (Chen S, J. M. Patel and E. R. Block (2000) “Angiotensin IV-mediated pulmonary artery vasorelaxation is due to endothelial intracellular calcium release” Am. J. Physiol. Lung Cell Mol. Physiol. 279: L849-L856; Marletta, M. A. (1993) “Nitric oxide synthase structure and mechanism” J. Biol. Chem. 268:12231-12234). Endothelial cell release of NO is enhanced by a number of receptor-mediated agonists including bradykinin, acetylcholine, histamine, angiotensin IV, and serotonin via signal transduction-mediated activation of eNOS (Chen, S., J. M. Patel and E. R. Block (2000) “Angiotensin IV-mediated pulmonary artery vasorelaxation is due to endothelial intracellular calcium release” Am. J. Physiol. Lung Cell Mol. Physiol. 279: L849-L856; and Davis M G, G. J. Fulton and P. O. Hagen (1995) “Clinical biology of nitric oxide” Br. J. Surg. 82:1598-1610.). The catalytic activity of eNOS is regulated by multiple post-transcriptional mechanisms involving a variety of factors including phosphorylation state, calcium mobilization, and protein:protein interaction (Patel, J. M. et al. (1999) “Increased expression of calreticulin is linked to Ang-IV-mediated activation of lung endothelial NOS” Am. J. Physiol Lung Cell Mol. Physiol. 277:L794-L801 and Garcia-Cardena G. et al. (1998) “Dynamic activation of endothelial nitric oxide synthase by HSP 90” Nature 392:821-824).
The cellular level cGMP is critical for the regulation of multiple functions including vasorelaxation. A family of enzymes known as phosphodiestreases (PDE) is known to control transient and sustained elevation of cellular cGMP levels through modulation of hydrolysis activity. PDE-5 is the predominant cGMP-hydrolyzing PDE isoform in pulmonary vasculature. Zaprinast, a cGMP-specific PDE inhibitor, can enhance vasorelaxation in pulmonary circulation (Schutte, H., M. Witzenrath, K. Mayer, N. Weissmann, A. Schell, S. Rosseau, W. Seeger, and F. Grimminger (2000) “The PDE inhibitor Zaprinast enhances NO-mediated protection against vascular leakage in reperfused lungs” Am. J. Physiol. Lung Cell Mol. Physiol. 279:L496-L502).
With regard to PDE inhibitors, there are several non-peptide therapeutic agents specific for inhibition of selective isoforms of PDE's, one of which is approved or used in human studies for the regulation of pulmonary vascular function in the US. Although Zaprinast is a selective inhibitor of PDE 5, a dominant isoform in the pulmonary vasculature endothelium, its pharmacologic effects have only been tested in an animal model. In addition, none of these agents are known to modulate cellular levels of cGMP through eNOS activation and NO production. Their physiologic action is exclusively based on reduced hydrolysis of cGMP.
Pulmonary arterial hypertension (PAH) is characterized by vascular obstruction and sustained elevation of pulmonary pressure or vasoconstriction. PAH is classified as primary (idiopathic) or secondary, associated with collagen vascular diseases.
Although the pathogenesis of PAH is not well understood, a number of intrinsic and extrinsic factors play a role in causing imbalance in the generation of vasodilators and vasoconstrictors in the circulation. This can lead to the development of histologic lesions including pulmonary arteriolar occlusion (Farber, H. W. and J. Loscalzo (2004) “Mechanism of Disease: Pulmonary arterial hypertension” N. Engl. J. Med. 351:1655-1665, 2004 and Newman, J. H., B. L. Fanburg, S. L. Archer et al. (2004) “Pulmonary arterial hypertension: future directions” Circulation 109:2947-2952).
At present, therapeutic approaches for treatment of PAH include various prostacyclin formulations (iloprost, treprostinil, beraprost), supplemental oxygen, anticoagulant (warfarin), calcium channel blockers, endothelin antagonists (sitaxsentan), supplemental nitric oxide (NO), and phosphodiesterase inhibitors (zaprinast, sildenafil) with limited success (Galie, N., A. Manes, and A. Branzi (2003) “Prostanoids for pulmonary arterial hypertension” Am. J. Respir. Med. 2:123-137; and Mehta, S. (2004) “Drug therapy for pulmonary arterial hypertension: what's on the menu today?” Chest 124:2045-2049).
There is a need in the art for new composition and methods to help regulate blood flow in order to address various pathological conditions as well as to improve cognitive function.